Seems like a random thing to call it though – why not "table" or "apple" – why "let"?

I'm not too sure of this. I remember reading somewhere(don't remember the source) that the term let comes from the french word for net, filet (fillet?). Initially it was only for serves that hit the net and landed inside. Later it was expanded to include more things like niff mentions(I am guessing this was done just so that another term need not be introduced )

In your opinion, it is better to be commitment or non-commitment player ? What are pros and cons ?

it's better to be a commitment player for the money because you can take part in the share of the ATP's "bonus pool", and also you're guaranteed to be allowed to take part in any ATP500 tournament for a year, even if your rankings drops out of the last entry ranking.

Unil 2011, it was less good to be a commitment player for the rankings because you had to count 4 or sometimes 3 ATP500 results or Monte-Carlo or Davis cup in your rankings.

But the new rule since 2012 makes that it's hardly a problem anymore, except if you get injured : the rule should really be adapted in that case from what it is now imo (see Monfils' case in 2012 who couldn't play any ATP500 tournaments then has 4 zeropointers in his rankings among his optional results, and will keep some of them until october ; Del Potro had the same problem when he came back after his wrist injury).